Pedicle screw systems have been in common practice for fusion procedures of the spine for several decades. The screw systems are placed using a posterior approach, either open or minimally invasive, in all areas of the spine from C1 to the sacrum.
The pedicle screw systems are typically deployed bi-laterally but can be used unilaterally. The purpose of the pedicle screw systems is to immobilize one or more motion segments, adjacent vertebral bodies and inter-vertebral disc. The immobilization is typically done as part of a fusion procedure.
These immobilizing constructs can bridge as few as one motion segment as would be typical for a one level fusion or many levels as would be typical for treating conditions such as scoliosis.
A typical pedicle screw system consists of a long bone screw that is placed through the pedicle and into the vertebral body on one or both sides of a vertebral body. This screw is typically terminated proximally with a mechanism for capturing and holding a rod that runs longitudinally between all of the pedicle screws employed, one rod on each side of the vertebral body.
The rods are typically manufactured with a straight configuration. Either during or prior to the surgical procedure, the rods are formed by the surgeon into a three dimensionally “bent” rod that will conform, as best as can be approximated, to the curvature and angulation of the spine. The rods are generally inserted into a “U” shaped receiver at the terminal end of the pedicle screw, the receiver having the ability to swivel, pivot and rotate, i.e., “polyaxial” so as to accept and capture the formed rod.
A set-type screw is threaded into the proximal end of the U receiver and tightened against the rod, thus holding it in place. Once fully in place, the pedicle screw construct will immobilize the treated motion segment(s) thereby creating a proper environment for the treated motion segments to fuse.
Currently, pedicle screw systems tend to be fairly bulky with a significant amount of screw and locking mechanism protruding beyond the vertebral body. This protrusion can irritate muscle tissue and may prove to be an annoyance to the patient. Implantation of the screws can be difficult for the surgeon as positioning each screw and the longitudinal rod(s) involves a great deal of steps.
Perhaps most critical is the degree of mechanical compromise inherent in traditional pedicle screw constructs. The “open” configuration is a compromise, mechanically, to accommodate in situ preparation and assembly of the pedicle screw construct. When compared to an “ideal” closed system wherein the rod is completely encircled and captured into the terminal end of the pedicle screw is inherently weaker and less rigid. Fully closed systems are not practical however, as anatomical access, variability from patient to patient and unique hand work of surgery prevent their application.